Leisure battery keeps dropping to 11.8v overnight — is the BMS giving up?

Been through something similar with my motorhome build last winter. Before condemning the BMS, worth ruling out a few things first.

11.8v overnight suggests one of three culprits:

• Parasitic draw you haven't accounted for (even 200–300mA adds up over 10 hours)
• A weak or failing cell dragging the pack down before the BMS cuts it
• The BMS itself triggering a low-cell disconnect and the voltage you're reading is resting on the wrong side of that

First thing I'd do is grab a clamp meter on your negative cable overnight and see what's actually drawing. My setup had a Victron BMV-712 which made this dead easy to spot — showed me a fridge compressor that wasn't cycling off properly.

If the draw looks fine, check whether 11.8v is a hard disconnect point on your BMS. Some cheaper units (I've seen this on a few non-branded packs) have low-voltage cutoffs set surprisingly high from factory.

What battery chemistry are you running? Lead-acid sitting at 11.8v is genuinely worrying — that's probably 20–30% SOC. LiFePO4 at 11.8v under no load would be extremely unusual and almost certainly means BMS intervention rather than natural discharge.

Also — how old is the pack? If it's a Fogstar or similar lithium unit, they'll often help diagnose via email if you send them a BMS log.

What's your setup — solar only, shore power, split charge? Others on here will have more targeted advice if we know the full picture.

@LDVAdventure decent list but I'd push back slightly on jumping to 11.8v being a BMS issue straightaway. That voltage on a lithium is basically full stop — it's already at the bottom of the usable range or below it depending on your chemistry.

What type of leisure battery are we actually talking about here? Because 11.8v means something completely different on a lead-acid (annoying but not dead) versus LiFePO4 (something's properly wrong, possibly BMS cutoff has already triggered and it's bounced back slightly).

On my static caravan setup I had something similar — turned out to be a dodgy Victron SmartShunt reading rather than actual cell voltage. Grabbed a proper multimeter directly on the terminals and the numbers told a different story entirely.

What's your monitoring situation? App, shunt, just a panel gauge?

@DODGuy fair point, and honestly 11.8v overnight on a lithium is more likely a capacity issue than BMS playing up. How old is the battery? My Fogstar 100ah was doing similar last year — turned out I'd been running it down too far too often and lost maybe 20% capacity without realising.

Worth doing a proper capacity test before anything else. Fully charge it, then put a known load on it and time how long it actually lasts vs what it should. Takes an afternoon but tells you exactly what you're working with.

Also — what's your resting consumption? Even a small 12v fridge pulling overnight adds up quick.

Good points from @DODGuy and @ThingamyBob62 already. One thing worth adding — what's your actual load overnight? Even a small fridge compressor cycling every few minutes will absolutely hammer a battery that's lost capacity. I'd stick a clamp meter on the negative terminal before bed and log what's actually being drawn. Also, 11.8v on a lithium isn't just low, it's genuinely concerning — most LiFePO4 cells are pretty much on the floor by then. If the BMS were at fault you'd typically see a sudden disconnect rather than a gradual voltage drop, so that to me suggests either parasitic drain or the cells genuinely can't hold charge anymore. How old is the battery and do you know the cell chemistry?

Jumping in to agree with @JoeFox55 on the load question — that's often where people get caught out. Worth also checking whether 11.8v is the resting voltage after loads are disconnected, or if you're reading it under load. Big difference diagnostically. A lithium sitting at 11.8v genuinely at rest is telling a very different story to one reading that whilst powering a fridge compressor mid-cycle. Give it 30 minutes completely isolated and recheck. Also, what chemistry are you running? LiFePO4 should sit comfortably above 13v at rest unless significantly discharged, so 11.8v either way is worth taking seriously regardless of whether the BMS is the villain here.